THE ORGANISMS IN YEAST. 221 



the fact which Prof. Lister has brought forward — viz., that the B. 

 ladis (the lactic ferment) refuses to live in Pasteur's " Cultivating 

 solution," and also by the fact, that while the yeast-cell only lives 

 with difficulty in a solution of sugar alone, and is not able to mul- 

 tiply in such a medium, similar conditions are actually destructive 

 to the life of the more minute organisms which are usually also 

 present in commercial yeast, and that on this fact a practical pro- 

 cess of yeast purification has been based. Moreover, those organ- 

 isms develop most readily and most freely in the nutrient fluid to 

 which it is best adapted, and where a number of dissimilar fer- 

 ments are placed in an organic liquid, some will usually increase to 

 the exclusion, and sometimes even to the extinction, of those of 

 weaker growth, or those to which the nutriment is less suited. It 

 is, moreover, a well-known practical fact to brewers, that the 

 nature of the development of cell-life which takes place during and 

 after the fermentation of their worts, is largely dependent on the 

 character of the nitrogenous and carbo-hydrate constituents. It 

 is, then, beyond doubt necessary to choose the cultivating medium 

 with special reference to the type of organism we wish to culti- 

 vate. 



Liquids adapted to be the soil for the nurture of certain classes 

 of organisms have been devised by Cohn and by Raulin — the 

 latter having been utilised by Pasteur — while grape-juice, malt 

 infusions, and various animal secretions, have been employed by 

 various observers according to the nature of their studies. For 

 such investigations as we are now considering, I have already 

 mentioned that an infusion of malt affords a convenient and suit- 

 able medium, and it may, if necessary, be supplemented by other 

 preparations, such as the nutrient fluid of Cohn, if it be desired to 

 cultivate separately the more minute organisms which are usually 

 associated with the germ-cells themselves. The malt from which 

 the infusion is made should be of the best quality procurable ; by 

 preference a malt made from barley grown and ripened in a hot 

 country ; or where a good brewer's wort can be obtained, this may 

 be substituted, diluting it to a standard strength — some 40? to 50'' 

 specific gravity will answer the purpose — with distilled water. This 

 infusion being placed in a number of cultivating-flasks, and boiled 

 as before described, is placed on one side for a time to ensure the 



